This invention relates to a gas sensor installed in an exhaust passage or the like of an internal combustion engine to measure the concentration of NOx or a specific gas component contained in the exhaust gas.
To measure a NOx concentration, an oxygen concentration, or an air-fuel ratio in an engine combustion chamber, a gas sensor is generally installed in an exhaust system of an automotive engine. For example, the gas sensor includes a gas sensing element arranged in the following manner.
The gas sensing element includes a pump cell for pumping oxygen with respect to the measured gas chamber and a sensor cell for measuring the concentration of NOx introduced into the measured gas chamber.
The sensor cell, measuring the NOx concentration, includes a pair of sensor electrodes and a solid electrolytic substrate having surfaces on which the sensor electrodes are formed. One of the sensor electrodes is positioned in a measured gas chamber. The other sensor electrode is positioned in an air chamber into which the air is introduced as a reference gas. The sensor electrode exposed to a measured gas is made of a material having active nature against NOx.
Furthermore, the pump cell includes a pair of pump electrodes and a solid electrolytic substrate having surfaces on which the pump electrodes are formed. One of the pump electrodes is positioned in the measured gas chamber. The pump electrode exposed to the measured gas is made of a material having inactive nature against NOx.
The measurement of the NOx concentration in the sensor cell is performed based on an oxygen ion current generating due to decomposition of NOx on the measured gas side electrode. Accordingly, the oxygen concentration in the measured gas chamber must be very small or stationary.
To this end, the pump cell is used to adjust the oxygen concentration in the measured gas chamber. The pump electrode is necessarily provided to be exposed to the measured gas. Decomposition of NOx, if happens on the pump electrode, will give adverse influence to the measurement of NOx concentration at the sensor cell. The measured NOx concentration will be inaccurate in this case.
To solve this problem, a conventional method is to add an inactive additive, such as Au, to a Pt pump electrode of a pump cell. Alternatively, a conventional pump cell employs an alloy pump electrode. This is effective to suppress the decomposition of NOx.
A monitor cell may be provided adjacently to the measured gas chamber to monitor the oxygen concentration in the measured gas chamber. Decomposition of NOx, if happens on a monitor electrode, will give adverse influence to the measurement of NOx concentration at the sensor cell. The measured NOx concentration will be inaccurate in this case, too.
Hence, to solve this problem, a conventional monitor cell employs a Pt electrode containing an inactive additive, such as Au, or an alloy electrode. Using such an electrode makes it possible to suppress the decomposition of NOx.
However, when the gas sensing element is exposed to a high-temperature exhaust gas, Au components volatilize from the pump electrode or the monitor cell electrode and then deposit or adhere on the measured gas side electrode of the sensor cell. This causes a serious change in the properties of the measured gas side electrode and deteriorates the accuracy in the NOx concentration measurement.